In sewage stations, septic tanks, wells, etc., it often occur that solid matter or pollutants, such as socks, sanitary pads, paper, etc., clogs the submersible pump that is lowered into the basin of the system. The contaminations are sometimes too big to pass through the pump if the impeller and the impeller seat are located at a fixed distance from each other. A way of solving the problem of solid matter clogging the pump is to admit the impeller to be movable in the axial direction in relation to the seal housing cover and the impeller seat, in order to form a momentarily bigger passage through the pump.
Usually the impeller is connected to a drive shaft extending from an engine located in a sealed off compartment of the pump. Thus, a seal is arranged between the drive shaft and the seal housing cover preventing liquid from entering the sealed off engine compartment. Adjacent to said seal is a cavity defined by the seal housing cover and the top surface of the impeller, which cavity communicates with the pump chamber of the pump via an opening gap. It is crucial that the opening gap is as small as possible, because if solid matter enters into the cavity it may damage the seal. However, at the same time the liquid shall be able to flow into and out from the cavity in order to cool down and lubricate said seal.
A decisive problem arises in the case when the impeller is movable in relation to the seal housing cover, and accordingly the volume of the cavity defined by the seal housing cover and the impeller is changed, i.e. decreased or increased. At the same time said cavity has to be bigger than for pumps having a stationary impeller, to allow a proper movement of the impeller. Most of the fluid contained in the cavity at the time the impeller starts to move upwards shall be pressed out through the small opening gap, and this takes place during seconds or parts of seconds. Thereby a great pressure peak arises in the cavity and the risk of damaging the seal is increased.
If said seal is damaged the engine and the entire pump may be damaged, and such an unintentional shutdown is costly, due to expensive, cumbersome and unplanned maintenance work.
A closely related Swedish Patent Application, SE 0501542-5, directed to the applicant, shows a pump for pumping contaminated liquid including solid matter. The pump comprises a rotatable impeller, which is movable in the axial direction in relation to the seal housing cover, or pump housing, between a first position and a second position. However, the above mentioned patent application does not discuss the abovementioned problems.
Furthermore, submergible pumps are used to pump fluid from basins that are hard to get access to for maintenance and the pumps often operate for long periods of time, not infrequently up to 12 hours a day or more. Therefore it is highly desirable to provide a pump having long durability.